Of course OS X is fairly secure, but I think all of this publicity surrounding its security has a bit of smoke-and-mirrors to it. Security holes are found, posted, and exploited in various libraries, apps, and OSes every day; the real question is always how quickly the problem gets addressed.
Apple has to find the right balance between over-frequent security updates, which is often perceived as nothing so much as an admission of security weaknesses on the part of the vendor (witness our general perception of Windows XP as insecure not only because of its well-publicized exploits, but also because of the frequency of the security updates), and under-frequent updates, leaving the user base with the feeling that Apple's trying to bury its problems.
This will become more of an issue as the OS gains popularity, and subsequently becomes a higher-value target for malicious hackers. I think Apple's been good about it so far.